The invention relates to an apparatus and a method for checking value documents marked with feature substances, and to the corresponding feature substances.
For safeguarding objects or in particular value documents such as bank notes, checks, passports, cards, etc. against forgeries, for quite some time here have been applied or incorporated feature substances as security features, whose presence can be proven by measuring their characteristic properties and employed for the authentication of the value document. A special class among these are machine-readable security features which, for example, are based on magnetic, photo-luminescent, electric-luminescent, optical absorption or further spectroscopic properties of the feature substances. In most cases, these cannot be recognized with the naked eye, but for authentication are detected and checked with a sensor specialized therein. In particular in the field of checking authenticity and state of bank notes by machine, in bank note processing machines these are guided past a series of sensors with high speeds, e.g. more than 11 m/s, and are automatically checked, evaluated and sorted.
In doing so, it is desirable that not only one measuring point per value document is obtained, but the total value document is scanned in spatially resolved manner. This increases the security of the authenticity check, on the one hand, because with an appropriate spatial extent of the security marking on the value document, also composite forgeries can then be recognized. Such forgeries are composed of pieces of different segments or portions of authentic value documents and other forged segments.
Furthermore, with a spatially resolved detection of a spatially locally attached security marking not only its general presence can be proven, but also its correct positioning on the document can be checked.
With a spatially resolved detection there can be additionally read out, due to the recognition of a security marking spatially attached e.g. in the form of a bar code, additional information about e.g. kind, series, value etc. of the value document.
In the literature there can be found several descriptions for marking value documents with Raman-active matter or in particular surface-enhanced Raman-scattering (SERS) active matter as well as their detection for securing the authenticity.
For example, WO 91/119492 A1 describes the safeguarding of value documents with a printing ink with fine-grained Raman-active components. These are proven upon irradiation with laser light of only few mW with the help of their characteristic Raman signal at powers to be detected in the pW region.
In EP 0806460 B1, the marking and safeguarding of value documents with a security ink containing SERS or surface-enhanced resonant Raman-scattering (SERRS) active matter is described. In so doing, the relatively weak Raman signals of the marker matters are often drastically amplified, by e.g. 6 orders of magnitude, by surface plasmonic effects occurring on the surface of metallic nanoparticles (SERS). In the case of the resonant Raman scattering the excitation wavelength is put spectrally in the vicinity of an electronic transition of the marker substance to be proven, which due to the resulting resonance entails a further significant amplification of the Raman signal. For the proof there are used classical instruments for stationary Raman spectroscopy in a microscope construction. Although these offer in principle an excellent spatial resolution, they have the serious disadvantage that typical measurement durations in the range of seconds up to several minutes are very long and hence are unsuitable for fast moved substrates.
EP 1385637 B1 also describes machine-readable security markings for value documents having a plurality of suitable marker substances or Raman-active molecules. For the authenticity proof there is employed, besides conventional commercial Raman spectrometers, a special portable Raman sensor with excitation in the infrared region and a CCD detector with 2048 pixels of linear resolution. Recording durations with integration times of 5 s to 60 s are achieved.
In WO 2014/022330 A2 SERS nanomarkers for securing the authenticity and the proof thereof with a confocal Raman microscope or a portable Raman spectrometer are described. In both cases the measurement duration of 5 s or 20 s is far too slow for being suitable for fast moved samples.
In WO 2007/146753 A2 there is described an in principle universally employable high speed Raman spectrometer which with an optomechanical rotary time-division multiplexer directs the individual wavelength channels respectively defined by a tailored bandpass spectral filter in a time shifted manner to one single very quickly readable detector. By this approach spectra within 1-100 ms can be obtained. This, however, offers no sufficient spatial resolution for fast moved samples, particularly because the here obtained spectra are spatially/spectrally smeared. Due to the staggered-in-time detection of the different spectral channels the intensity information of the different spectral channels does not come from the same place, but depending on the detection time of the specific channel from a place further away on the sample moved further.
In WO 2012/030988 A1 there is described an inline spectral sensor for SERS- or Raman-marked moved objects. This includes a fiber-coupled measurement head with connection to the light source as well as to the spectrometer which is equipped with a CCD-based detector. The spectral resolution here is to be between 0.01 nm and 5000 nm. A measuring time for recording a spectrum at an SERS-marked document of 5 ms at a speed of 10 m/s is achieved, which—if immediately repeatable—corresponds to a spatial resolution of at best 5 cm. This is not sufficient for an authenticity determination with position check of the marking substance on the value document or for a recognition of bar codes.
For the machine check of the authenticity of moved luminescent value documents, such as for example bank notes, there are further known sensors, e.g. from WO 2006/010537 A1, which check the luminescence properties of marking substances attached thereto spectroscopically. However, these are not suitable for measuring Raman or SERS signals. The Raman signals occur in a spectral region which with a typical shift of possibly only few 1-100 cm−1 is very near to the excitation laser wavelength and—in contrast to luminescent signals—cannot be separated in a time-resolved manner via an afterglow. Moreover, the intensity of the Raman lines is very small in comparison to the Rayleigh line of the elastically scattered excitation light. Therefore, the existing luminescence sensors are not suitable for the proof of Raman markers, because here the rather weak Raman signals are completely over-irradiated by the Rayleigh-scattered excitation light during the excitation.
Furthermore, imaging and thus spatially resolving sensors for the machine check of moved bank notes are known, for example, from WO 96/36021 A1. These achieve, by recording the light reflected at the bank note via a filtering and recording with line CCD cameras, a spatial resolution smaller than 0.5 mm at speeds higher than 5 m/s. Such cameras, however, are not suitable for the secure identification of Raman-spectroscopy signals, because, firstly, the illumination is unsuitable for generating useful Raman signals, secondly, the necessary spectral resolution with regard to selectivity as well as number of channels is far from being achievable, and, thirdly, the necessary filtering of the intensive excitation light with a suppression by many orders of magnitude is not representable in such an image sensor geometry.
Summing up, the safeguarding of value documents with Raman or SERS marking substances as well as their basic proof by stationary Raman spectroscopy or also by Raman spectroscopy carried out with moved value documents is known. However, there exist no sensors which are able to render the specific proof of a Raman or SERS marker over the document at least along a track in a spatially resolved manner also at transport speeds usual in bank note processing machines of up to 11 m/s or more.